A base station transmits, during a first period, a first plurality of voice packets of a first talking period on a first plurality of subcarriers of a first carrier. The base station transmits, during a second period, a second plurality of voice packets of a second talking period on a second plurality of subcarriers of a second carrier. The base station transmits, in the first period and the second period, data traffic packets on a third plurality of subcarriers. There is at least one guard band between at least two subcarriers in the third plurality of subcarriers.